ACCIONA used Danfoss technology to implement an innovative reverse osmosis modular desalination system at the Sarlux refinery in Sardinia. As a result, energy consumption was reduced (from 2.7 kW to 2,4 kW per m3 of water) in comparison with traditional solutions. The process of desalination is intrinsically energy intensive. Therefore, operating costs related to electricity consumption are very significant and have a direct impact on the sustainability and profitability of a desalination plant. However, technological evolution can change the rules of the game. Nowadays we have more advanced high pressure pumps, more efficient isobaric energy recovery systems in comparison with conventional Pelton turbines, new Low Energy reverse osmosis membranes and better electronic control using VLT® variable drives. Thanks to these technologies, next generation reverse osmosis modular systems for industrial applications are economically viable, as shown by the successful project implemented by ACCIONA at the Sarlux refinery and power station in Sardinia.
Guaranteed and cost-effective water supply
According to Pietro Tota, D & C Manager at ACCIONA Agua: "Sarlux was considering upgrading the process used to obtain ultra pure water for its industrial plants, and we proposed to them an innovative desalination solution based on reverse osmosis. This solution can be carried out in containers and offers the client high availability, flexibility, low energy consumption, and is economically sustainable."
ACCIONA used Danfoss technology to implement an innovative reverse osmosis modular desalination system at the Sarlux refinery in Sardinia
The operation of the refinery and adjacent power station in Sarroch requires an average of 1,000 m3 of water every hour. Previously, Sarlux obtained water from a local canal and different desalination plants based on traditional multi-stage flash technology with evaporation phase, with high energy use, about 20 kWh per m3 of water produced.
Thanks to its international experience in the area of desalination by reverse osmosis, ACCIONA proposed a comprehensive solution to Sarlux. The turnkey contract for six years, with a possible three-year extension, guarantees the supply of ultra pure water based on an all inclusive rate which is economically competitive when compared with existing alternatives.
As Tota stated: "We were in charge of everything, from planning the project proposal to its financing, from the project design to construction, and now we are in charge of operating it, as well as ordinary and extraordinary maintenance."
The solution designed by ACCIONA includes four reverse osmosis desalination modular lines with a production capacity of 12,000 m3 of pure water per day; the guaranteed availability is 97% throughout the year. The structure of each modular line is similar, and comprises four main sections: pretreatment with ultrafiltration membranes, first stage of reverse osmosis with working pressures of about 60 bar, second stage of reverse osmosis to refine water and a last stage with ion exchange membranes.
Innovative technology that makes a difference
The Danfoss axial piston high pressure pumps, together with the iSave Danfoss isobaric energy recovery devices, have shown to be ideal to implement a modular system in the container provided by ACCIONA for the refinery in Sarroch.
Tota explains it further: "The technology of Danfoss high pressure pumps allows systems that not only are energy efficient, but also extremely compact, and they can be placed in a container. Thanks to the small size and simple internal design, they can be installed even in confined spaces, and also they require little regular maintenance, something as equally important. iSave devices capture hydraulic energy from the reject stream of the reverse osmosis process, drastically reducing energy consumption, and are also extremely compact and can be integrated with the Danfoss high pressure pumps."
Another essential component to minimise energy consumption are frequency converters to regulate flow.
We are talking about the D frame VLT® AQUA Drive product line, with 200 kW of power, extremely powerful and compact. They are 68% more compact than previous versions, for 90 to 315 kW of power, and feature RFI filters as a standard in the intermediate circuit.
Tota adds: 'Clearly the VLT® AQUA Drive has been specifically designed to control pumps, including ad hoc features as a standard, and its use is simple and intuitive'.
Thanks to the combined design of the flow regulation and control systems done by ACCIONA and Danfoss, we have obtained a solution able to improve energy efficiency under any operating conditions, depending on flow, environmental conditions and the level of salinity of seawater.
Ernesto Ubieto, President of South Europe Region, Danfoss, explains: 'Danfoss has the four main technologies for desalination plants: high pressure pumps, energy recovery systems, variable speed drives, and pressure and flow control elements. The key challenge of desalination is that the process requires a lot of energy. Therefore, experience, technology and innovation should be the focus of any value proposition related to desalination. The ACCIONA desalination plant received a 'Distinction Award' in Paris during the 2018 Global Water Intelligence Awards, which recognise the best initiatives in the water, waste water and desalination sectors adopting innovative technologies and sustainable financial models. The commissioning of the new desalination plant will allow dismantling some desalination sections built in the 1990s which are no longer energy efficient'.